The atmospheric longevity of carbon dioxide

How long does carbon dioxide emitted by human beings remain in the atmosphere? It turns out, it is a tricky question. Different mechanisms remove carbon at different rates, and the responses of each system to higher concentrations of carbon dioxide in the atmosphere differ.

Probably the most important distinction is between sinks that have a capacity that can be exhausted and those that are effectively limitless. Oceans the biosphere are of the first kind, and they respond to carbon dioxide in the atmosphere relatively quickly. That being said, there is a limit to how much carbon dioxide the ocean can absorb (and the fact that it becomes more acidic while doing so is problematic) and there is only so much biomass the planet can sustain. Weathering rock that absorbs carbon and then subducts below the seafloor is an example of the second type of sink: though it operates very slowly and volcanic eruptions can return carbon that has been locked into the lithosphere back to the atmosphere. Even without such eruptions to worry about, natural weathering is not the route to a stable climate on a human timescale. As the Nature article linked above explains: “it would take hundreds of thousands of years for these processes to bring CO2 levels back to pre-industrial values.”

The article also comments on how long the temperature anomaly from anthropogenic emissions will persist: “whether we emit a lot or a little bit of CO2, temperatures will quickly rise and plateau, dropping by only about 1°C over 12,000 years.” We should make no mistake in understanding that our choices about how much carbon dioxide we emit will have a big impact on a huge number of future generations.

WASHINGTON — Many damaging effects of climate change are already basically irreversible, researchers declared Monday, warning that even if carbon emissions can somehow be halted temperatures around the globe will remain high until at least the year 3000.

“People have imagined that if we stopped emitting carbon dioxide the climate would go back to normal in 100 years, 200 years – that’s not true,” climate researcher Susan Solomon said in a teleconference.

Ms. Solomon, of the National Oceanic and Atmospheric Administration’s Earth System Research Laboratory in Boulder, Colo., is lead author of an international team’s paper reporting irreversible damage from climate change, being published in Tuesday’s edition of Proceedings of the National Academy of Sciences.

Study shows that anthropogenic CO2 and the resulting climate change are longer lived than previously thought. Even after 10,000 years, 15% to30% of the anthropogenic CO2 perturbation persists in the atmosphere. The resulting climate changes last even longer.-

Thus, with a 3% increase in emissions per year and 50% of that remaining airborne, it is easy to compute the increase in CO2 concentrations. He obtains an increase from 295 to 403 ppm from 1900 to 2010. The actual value in 2010 is 390 ppm, a little lower than Broecker estimated because his forecast cumulative emissions were a little too high.”

“Yes, there can be natural climate changes over thousands or millions of years that are large compared to what we are experiencing now. But in fact, our actions have risen well above the level of any natural variations because of their pace. Without our use of fossil fuels, we should be descending into another glacial maximum – albeit slowly, over tens of thousands of years (the pace of natural global climate change in the Pleistocene). And instead our actions have interfered with this natural cycle and we are in the process of completely deglaciating the planet.

New research indicates the impact of rising CO2 levels in the Earth’s atmosphere will cause unstoppable effects to the climate for at least the next 1000 years, causing researchers to estimate a collapse of the West Antarctic ice sheet by the year 3000, and an eventual rise in the global sea level of at least four metres. The study, to be published in the Jan. 9 Advanced Online Publication of the journal Nature Geoscience, is the first full climate model simulation to make predictions out to 1000 years from now. It is based on best-case, ‘zero-emissions’ scenarios constructed by a team of researchers from the Canadian Centre for Climate Modelling and Analysis (an Environment Canada research lab at the University of Victoria) and the University of Calgary.

“We created ‘what if’ scenarios,” says Dr. Shawn Marshall, Canada Research Chair in Climate Change and University of Calgary geography professor. “What if we completely stopped using fossil fuels and put no more CO2 in the atmosphere? How long would it then take to reverse current climate change trends and will things first become worse?” The research team explored zero-emissions scenarios beginning in 2010 and in 2100.

THE natural processes which dispose of carbon dioxide are, in aggregate, rather slow, which means that an increase in the atmosphere’s carbon-dioxide level will, left to itself, last a long time. David Victor, a professor at the University of California, San Diego, has a keen eye for such simple basics and the uncomfortable ways they may fit together. He sees this one as underlying all three of the things that make climate change a particularly pernicious sort of problem.

The global carbon cycle involves a constant exchange between oceans, biota, and the atmosphere. A large portion of the carbon cycle, however, extends deep into Earth’s solid interior, and exchange occurs over much longer time scales. Walter et al. (p. 54, published online 15 September; see the Perspective by Harte) show evidence, through the analysis of a unique set of Brazilian diamonds, that the deep carbon cycle extends further into Earth than previously anticipated. The isotopic signature of diamonds suggests that they formed from carbon that originated from subducted oceanic crust, but tiny mineral inclusions trapped within the diamonds reveal that they must have passed through the lower mantle before being sent back up to Earth’s surface.

Science 10 February 2012:
Vol. 335 no. 6069 p. 655
DOI: 10.1126/science.335.6069.655-a
Letters
Carbon Shifted But Not Sequestered
The News & Analysis story “An unsung carbon sink” (C. Larson, 18 November 2011, p. 886) states that the erosion of limestone by carbonic acid formed from atmospheric carbon dioxide may constitute an “underappreciated carbon sink,” partially mitigating the increase of carbon dioxide from anthropogenic sources. This mitigation is only possible if the captured carbon dioxide is sequestered in an unreactive form. In this case, it is not.

Weathering of limestone consumes carbon dioxide to form soluble bicarbonates in solution. If redeposited as calcite (e.g., as travertine or speleothems), the associated carbon dioxide will be returned to the atmosphere. If carried into the oceans, it may be sequestered as shells or reefs, again with the release of the carbon dioxide. There will therefore be no net sequestration of carbon dioxide, but rather a transfer to the oceans, where it will equilibrate over time with the atmosphere.

July 26, 2013 — A study of a global warming event that happened 93 million years ago suggests that the Earth can recover from high carbon dioxide emissions faster than thought, but that this process takes around 300,000 years after emissions decline.Scientists from Oxford University studied rocks from locations including Beachy Head, near Eastbourne, and South Ferriby, North Lincolnshire, to investigate how chemical weathering of rocks ‘rebalanced’ the climate after vast amounts of carbon dioxide (CO2) were emitted during more than 10,000 years of volcanic eruptions

In chemical weathering CO2 from the atmosphere dissolved in rainwater reacts with rocks such as basalt or granite, dissolving them so that this atmospheric carbon then flows into the oceans, where a large proportion is ‘trapped’ in the bodies of marine organisms.

The team tested the idea that, as CO2 warms the planet, the reactions involved in chemical weathering speed up, causing more CO2 to be ‘locked away’, until, if CO2 emissions decline, the climate begins to cool again. The Oxford team looked at evidence from the ‘Ocean Anoxic Event 2’ in the Late Cretaceous when volcanic activity spewed around 10 gigatonnes of CO2 into the atmosphere every year for over 10,000 years. The researchers found that during this period chemical weathering increased, locking away more CO2 as the world warmed and enabling the Earth to stabilise to a cooler climate within 300,000 years, up to four times faster than previously thought.

The Ocean Anoxic Event 2 (OAE2) about 93.5 million years ago was marked by high atmospheric CO2 concentration, rapid global warming and marine anoxia and euxinia. The event lasted for about 440,000 years and led to habitat loss and mass extinction. The marine anoxia is thought to be linked to enhanced biological productivity, but it is unclear what triggered the increased production and what allowed the subsequent rapid climate recovery. Here we use lithium isotope measurements from carbonates spanning the interval including OAE2 to assess the role of silicate weathering. We find the lightest values of the Li isotope ratio (δ7Li) during OAE2, indicating high levels of weathering—and therefore atmospheric CO2 removal—which we attribute to an enhanced hydrological cycle. We use a geochemical model to simulate the evolution of δ7Li and the Ca, Sr and Os isotope tracers. Our simulations suggest a scenario in which the eruption of a large igneous province led to high atmospheric CO2 concentrations and rapid global warming, which initiated OAE2. The simulated warming was accompanied by a roughly 200,000 year pulse of accelerated weathering of mafic silicate rocks, which removed CO2 from the atmosphere. The weathering also delivered nutrients to the oceans that stimulated primary productivity. We suggest that this process, together with the burial of organic carbon, allowed the rapid recovery and stabilization from the greenhouse state.

This inert fossil fuel carbon inside us has no direct effect on our health, although mercury and other pollutants that often accompany it amid industrial and automotive emissions may harm us. Most of the airborne carbon will eventually dissolve into the oceans, leaving a sizable fraction of it aloft until it, too, is removed by chemical reactions with carbonate and silicate minerals in rocks and sediments.

That’s the good news.

The bad news is that the natural mopping up of our mess will be extremely slow. Research by the University of Chicago oceanographer and climate scientist David Archer and others shows that the cleanup will take tens of thousands of years even if we switch quickly to renewable energy sources. When the Earth’s slow cyclic tilting and wobbling along its eccentric orbital path once again leads to a major cooling period some 50,000 years from now, enough of our heat-trapping carbon emissions will still remain in the atmosphere to warm the planet just enough to weaken that chill. In other words, our impacts on global climate are so profound that we will have canceled the next ice age.

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This best-case scenario is troubling, but Earth history shows us that the alternative is unacceptable. If we burn all remaining coal, oil and gas reserves within the next century or two, we could introduce a more extreme, longer-lasting hothouse much like one that occurred about 56 million years ago: the Paleocene-Eocene Thermal Maximum, or PETM.

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Unlike the relatively mild interglacials driven by the tilt, wobble and orbit of the Earth, the PETM fundamentally transformed the planet. Experts speculate that it was set off by volcanism in the Atlantic Ocean, thawing of permafrost, melting of methane hydrates, or a combination of such factors. Whatever caused the PETM, it spewed trillions of tons of carbon dioxide into the air and oceans. Global average temperatures climbed 10 degrees or more, erasing cold-loving species and habitats from the planet. With atmospheric carbon dioxide concentrations several times higher than today, a combination of warming and carbonic acid buildups in the oceans exterminated many deep-sea creatures and dissolved limy minerals and shells from the ocean floor.